1. Field of the Invention
This invention relates to conductors for solid state devices and more particularly to multiple layer conductors.
2. Description of Prior Art
Magnetic bubble applications require thin film conductors to pass larger current densities than any other application in the microelectronics industry. Pulse current densities exceeding 1.times.10.sup.7 amperes/cm.sup.2 have been reported by Kinsbron et al, Proceedings of the 16th IEEE Reliability Physics Symp., April 1978, p. 248. Thus, metallurgies for thin film conductors in these applications must be exceptionally resistant to electromigration-induced failure.
Conductors serve a variety of functions in magnetic bubble applications, and the current density and the amount of operation the conductor receives is dependent on its function. Typical conductor functions and the type of usage received are listed in Table I.
TABLE I ______________________________________ Conductor Functions in Magnetic Bubble Devices Current Pulse Function Density Duty Duration ______________________________________ Bubble Gene- Very High Frequent Very Short rator (Nucleator) Data Switch High Frequent Moderate Transfer Switch High Infrequent Moderate Replicator High Frequent Moderate Bubble Stretcher Moderate Frequent Long ______________________________________ Legend: Current Density Very High (1 .times. 10.sup.7 amps/cm.sup.2) High (4 .times. 10.sup.6 amps/cm.sup.2) Moderate (2 .times. 10.sup.6 amps/cm.sup.2) Duty Frequent (e.g., every 4 .mu.sec.) Infrequent (e.g., every 400 .mu.sec.) Pulse Duration Very short (e.g., 40 nsec.) Moderate (e.g., 1-2 .mu.sec.) Long (e.g., 2-4 .mu.sec.) ______________________________________
In general, continuous (d.c.) currents are not encountered in magnetic bubble devices.
Unidirectional current pulses in the current density ranges used in bubble domain devices induce motion of the atoms comprising the thin film conductor, an effect known as electromigration. Electromigration induces crack or void formation in the conductor which, over a period of time, can result in conductor failure. The rate of electromigration is dependent on the conductor material, the current density imposed on the conductor and the conductor temperature. In high current density applications, potential conductor failure due to electromigration can severely limit the reliability of the circuit. Electromigration can cause an additional problem in magnetic bubble domain devices when a passivation layer such as glass, silicon nitride, or silicon dioxide is overlaid on the device, as is typically done in the industry. This layer can fracture as a result of the removal and build-up of conductor material. This fracture can expose conductors to the atmosphere which may cause corrosion.
Conventionally, in the microelectronics industry, aluminum has been used as the conductor material. For the current density requirements of magnetic bubble devices, aluminum conductors are not reliable because of their susceptibility to electromigration. Several metallurgical systems have been proposed previously in which electromigration occurs more slowly than in pure aluminum conductors, thus leading to longer conductor life or the ability to impose higher current densities on the conductor. Among the proposed metallurgical systems are AlCu, Ta/Au/Ta, Al-intermetallic configurations and Au-intermetallic configurations.
The basic requirements which a metal or metal system (i.e., a configuration of one or more metals, either as an alloy, pseudo-alloy, or layered structure) must achieve in order to be a suitable candidate for use as a thin film conductor in integrated, solid state circuits or magnetic bubble devices are summarized as follows:
(1) high electrical conductivity--only materials having high electrical conductivity are considered because low conductivity values lead to excessive Joule heating and too large a voltage loss in the conductor, or the alternative of making the conductor with an unacceptably large cross-sectional area.
(2) corrosion resistance--conductor materials which are susceptible to corrosion, even if they are relatively resistant to electromigration induced failure, are prone to fail from corrosion effects over the intended lifetime of the device.
(3) chemical stability with regard to other materials with which the conductor will be in contact in the magnetic bubble device; such materials are typically silicon dioxide, and ferromagnetic garnet materials such as rare earth substituted gallium or germanium iron garnets, e.g., EuTm.sub.1.26 Y.sub.0.48 Ga.sub.0.5 Fe.sub.4.5 O.sub.12.
(4) In some magnetic bubble applications, NiFe patterns are fabricated either crossing or directly on top of the thin film conductors. These NiFe patterns serve to direct the motion of the magnetic bubble domains in the bubble storage layer. The thin film conductor metal system must be compatible with the NiFe patterns. Two exemplary areas of concern are interdiffusion of the conductor metal system with the NiFe patterns and topography (specifically grain size and roughness) of the thin film conductor which influences the coercivity of the NiFe patterns. Larger coercivity is to be avoided because it requires excess magnetic energy for domain rotation, thus smooth, small grained films are preferred.
(5) For magnetic bubble applications, the conductor structure is required to be nonferromagnetic because a magnetic conductor would interfere with the directed motion of the magnetic bubble domains.
(6) adhesion to the substrate on which the thin film conductor is fabricated, and adhesion to the passivating layer applied over the thin film conductor. This passivating layer insulates the conductor and provides a degree of protection against corrosion. Thus, adhesion to silicon dioxide and possibly garnet materials is required.
(7) compatibility with later processing of the device. This compatibility may take the form of metallurgical stability under elevated temperature conditions encountered in processing, and resistance to chemical attack by certain chemical agents to which the conductor may become exposed in processing.
(8) ability to be deposited by the fabrication techniques in common use within the industry for metal deposition, for example by vapor deposition, sputter deposition, or electroplating.
(9) certain properties of the film's grain structure are also important. In order to obtain adequate line definition by a lift-off process, the film should be small grained, with a grain size not exceeding about one-third of the required linewidth. Uniformity of grain size and preferred crystallographic orientation of the grains are also factors which promote longer electromigration limited conductor lifetimes. Fine grained films are also smoother, which is a desirable quality in magnetic bubble applications to lessen difficulties associated with covering the conductor with an overlayer.
No single metal is exceptionally well suited to satisfying all of the above requirements. Aluminum, copper, silver, and gold provide high conductivity, but of them only Al adheres well to silicon dioxide and garnet films used in magnetic bubble devices.
Using metal systems (i.e., combinations of metals) significant progress has been achieved toward reducing the effects of electromigration in thin film conductors. A recent review of advances in using Al-based metal systems is provided in U.S. Pat. No. 4,017,890 of Howard et al which is commonly assigned.
Adhesion problems encountered in using Ag, Au or Cu conductors have been overcome by using adhesion layers between the conductor and the material to which it must adhere. However, Ag is susceptible to corrosion. Cu is also susceptible to corrosion, although to a lesser degree.
Au provides a high degree of resistance to electromigration induced failure, and to corrosion, is not attacked by most chemical agents, and is easily deposited in pure form by vapor deposition, sputter deposition, or electroplating. With proper techniques of deposition, it forms a small, uniform grain structure, with strong preferred orientation of the (111) crystallographic direction normal to the film plane. Au in conjunction with Ta adhesion layers has been proposed as an electromigration resistant metallurgy for semiconductor applications by Riseman et al in U.S. Pat. No. 3,617,816 which is also commonly assigned. Au in conjunction with adhesion layers of Nb, Hf, or Zr, which form intermetallic compounds with Au, has been proposed by Gangulee et al in U.S. Pat. No. 4,166,279, which is commonly assigned, wherein there is also a review of other advances in using Au-based metal systems.